This invention relates to ultrasonic wave generators, and in particular to a circuit for driving an ultrasonic transducer used for atomizing fuel oil over an extended temperature range with improved efficiency.
Numerous circuits which can be used to drive an ultrasonic transducer at useful power levels with reasonable efficiency are known. These transducers are commonly made from a piezoelectric ceramic material which exhibits electro-mechanical resonance effects typical of many piezoelectric devices. When operated at one of the natural resonance frequencies, greatly improved electrical to mechanical power conversion can be accomplished when the resulting vibrations are amplified using a suitable horn.
There are two basic ways to detect resonance of a piezoelectric transducer. Assuming the most common situation of driving from a constant voltage source, the frequency can be varied until a relative maximum amplitude of driving current is found. This is the series resonance frequency. Alternatively, if parallel resonance is desired, a relative current minimum is searched for. A means of eliminating the influence of the nominal capacitance of the transducer is required when operating at series resonance, such as adding a tuning inductor, otherwise the amplitude peak will not occur exactly at the true resonance frequency. With this method, the phase relationship between the transducer voltage and current is ignored.
The second basic method is to ignore the signal amplitude, and search for a frequency where transducer voltage and current are in phase. Since this occurs both at series and parallel resonance, the very large difference in transducer current for these two resonance modes may easily be used to differentiate between them. As before with the amplitude method, a means of tuning out the nominal transducer capacitance is required, in this case to ensure that the transducer is purely resistive at reasonance, and therefore that current and voltage are in phase at this point. With this method, other than to use the very large difference in transducer current at series compared with parallel resonance, signal amplitude is of no interest.
Both methods have advantages and disadvantages. Many of the recent patents on ultrasonic generators use the amplitude method for resonance detection. Although the basic concept is simple, this method suffers the very serious disadvantage that there is no absolute amplitude value to use as a reference for comparison, since this affected by many factors such as operating power level, tolerances of the transducer and of the generator circuit, loading of the transducer, etc. A relative comparison must be made of the signal level at different frequencies, on a continuous basis in order to find and follow the frequency which produces the highest amplitude. Thus, most of the patents which disclose amplitude-searching circuits, describe various ways of making small continuous frequency changes and keeping track of which frequency produces the highest relative amplitude. The simplicity of this basic concept is therefore complicated considerably. This method also suffers the disadvantage of greater noise sensitivity since most electrical noise affects signal amplitude, not frequency (the same reason that FM radio is far less affected by noise than AM radio).
The phase comparison method, by comparison is unaffected by signal amplitude variations; when driven at resonance, voltage and current are in phase regardless of amplitude. Another very major advantage is that the frequency is not required to be continously changed in order to search for the correct point of operation; the voltage and current signals are always present, and therefore can be continously compared to produce an error signal used to drive the circuit to the correct operating frequency. A disadvantage of this method is that it is not possible to tell the difference between series and parallel resonance, which must be accomplished separately by, for example, detecting the very large difference in amplitude between series and parallel resonance as mentioned above. The major problem with this method is that it is technically more difficult and does not lend itself well to the use of digital design techniques which are becoming more commonly used.
A known application of ultrasonic waves is in the atomization of liquids, particularly fuel oil. Specifically, a piezoelectric transducer is constructed so that fuel is allowed to flow over the surface of its horn. When the transducer is excited at one of its natural resonance modes with sufficient amplitude, the film of fuel oil that covers the horn is propelled from the surface in the form of a fog of fine droplets. Such an ultrasonic transducer has applications as a means of atomizing the fuel in an oil burning furnace, replacing, for example, the commonly used high pressure spray nozzle.
A disadvantage that occurs from the above mentioned operation of an ultrasonic transducer in a resonance mode, is that the sharp "Q" values obtained produce an attendant narrow operating frequency band. Relatively small deviations from the natural resonance frequency of the transducer can cause a significant reduction in power output. It is therefore necessary for the ultrasonic generator to track the natural resonance frequency of the transducer, which may not only change over time, but because of small and unavoidable differences between transducers, the resonance frequency may differ significantly between different transducers of the same type. The cause for differences in resonance frequency between apparently identical transducers is mainly tolerance differences, both in the dimensions of the mechanical parts and in the dimensions and electrical properties of the piezoelectric components. The causes for the change in resonance frequency over time include the known temperature dependence and ageing effects of piezoelectric elements, and specifically with ultrasonic atomizers, the additional mass of the liquid being atomized which may vary depending on conditions and type of liquid, and buildup of contaminants on the transducer such as carbon deposits.
The above mentioned tolerance differences also cause deviations in the characteristic impedance or reactance of transducers. Thus, when driven with a constant driving voltage, or with a constant driving current, apparently identical transducers produce different levels of output power. Some means is needed to ensure that the power output of all transducers is approximately equal.
A further problem specific to ultrasonic atomizers is the possibility of flooding the transducer horn with excess liquid. When this occurs, atomization stops and the otherwise sharp "Q" of the transducer is reduced to a very low value due to the damping action of the liquid, making it difficult to detect any resonance effects of the transducer.
The nature of ultrasonic atomization creates another problem. There is required a minimum amplitude of vibration before sufficient energy is imparted to the liquid on the transducer to cause it to be propelled from the horn.
A major influence on this minimum amplitude or power level required is the viscosity of the liquid being atomized. Specifically with fuel oil, although the minimum power level required for atomization is very low at normal temperatures, this minimum power level increases dramatically at low temperatures, and is significantly affected by fuel oil quality and type. Therefore, at low temperature, impractically high levels of power may have to be used to achieve atomization, due in large part to the increasing viscosity of the fuel oil at low temperature.